Various methods are well known for wavelength tuning of lasers. Typically the tuning takes place in a device referred to as a line narrowing package or line narrowing module. A typical technique used for line narrowing and tuning of excimer lasers is to provide a window at the back of the discharge cavity through which a portion of the laser beam passes into the line narrowing package. There, the portion of the beam is expanded and directed to a grating which reflects a narrow selected portion of the laser's broader spectrum back into the discharge chamber where it is amplified. The laser is typically tuned by changing the angle at which the beam illuminates the grating. This may be done by adjusting the position of the grating or providing a mirror adjustment in the beam path. The adjustment of the grating position or the mirror position may be made by a mechanism which we will refer to as a wavelength adjustment mechanism. For many applications it is important that the laser not only be finely tunable but also that the wavelength of the beam be set to a precise absolute value, with a very small deviation, such as for example 193.3500 nm.+-.0.0001 nm. This requires very precise calibration of the wavelength adjustment mechanism. Wavelength measurements are typically made using gratings and/or etalons which disperse a laser beam spectrally to produce a spatial intensity distribution which is a function of wavelength. These devices are typically able to determine only changes in wavelength; therefore, in order for these devices to be used to measure absolute wavelengths they must be calibrated using a known reference wavelength.
U.S. Pat. No. 5,450,207, entitled "Method and Apparatus for Calibrating a Laser Wavelength Control Mechanism," by Igor Fomenkov, assigned to the present assignee and incorporated herein by reference, describes a method for calibrating a wavelength adjustment mechanism for a KrF excimer laser. In the '207 patent, a small portion of the light emitted by a laser is passed through a cell containing FeNe vapor, used as an absorption gas. The light exiting this vapor is then detected by a photodetector, and the intensity of the detected light is then analyzed. The FeNe vapor absorbs a portion of the laser light at a wavelength of 248.3271 nm. The laser has a tunable range between 247.9 nm to 248.7 nm. The wavelength of the beam can be tuned anywhere in the range by pivoting a tuning mirror. One or more dips in the intensity of the laser light passing through the vapor as the mirror is slewed through a range of angles are detected and used to calibrate the laser wavelength measuring system (hereinafter called wavemeter) so that the wavemeter will indicate a wavelength of 248.3271 nn when the laser is tuned to the above FeNe absorption peak of 248.3271. The wavemeter, once calibrated, may then accurately measure the absolute wavelength of laser light at other wavelengths. Such a wavemeter is described in U.S. Pat. No. 5,025,445, assigned to the present assignee and incorporated herein by reference. The calibration unit is called the atomic wavelength reference unit or simply "AWR".
The National Institute of Standards and Technology has published emission lines of platinum at 193,224.33 pm and 193,436.9 pm.
If the laser is used in a stepper in a wafer fabrication system, the stepper optics and the fabrication process are optimized for a specific laser wavelength. Accordingly, it is important that the laser wavelength be adjusted accurately so that a maximum amount of the laser energy occurs at the desired wavelength. It is also important that the bandwidth be kept very small, in the range of about 0.6 pm and that reliable methods of measuring it be available. Also, if the reliability of bandwidth measurements are questionable, a method is needed to confirm that the measurement is accurate or not.
What is needed are alternate techniques for estimating the bandwidth of narrow band excimer lasers.